1. Field of the Invention
The present invention relates to a supply voltage control device for an envelope tracking amplifier. More particularly, the present invention is concerned with a supply voltage control device for an amplifier capable of feeding a supply voltage, which is necessary to a power amplifier, while sustaining high efficiency, and ensuring the linearity of the power amplifier.
2. Description of the Related Art
In recent digital wireless communication systems, a multilevel modulation method in which a mean power of a signal and a peak power thereof largely differ from each other is often adopted in order to raise a transmission speed.
In conventional power amplification systems, when such a signal is handled, a constant voltage making it possible to output the peak power is fed to a power amplifier in order to ensure the linearity of the power amplifier.
However, a time during which the peak power is outputted is very short. As a result, the power supply efficiency of the power amplifier is degraded.
As a technology for solving the foregoing problem, an envelope tracking type power amplification system is known.
FIG. 6 shows an example of a configuration of an existing envelope tracking type power amplification system.
The power amplification system of the example includes an envelope detector 101, an error amplifier 21, a resistor (R) 22, a pulse width modulator 23, a pulse amplifier 24, a coil (L) 25, a delay circuit (Delay) 1, and a power amplifier 2.
For convenience' sake, in FIG. 6, the same reference numerals are assigned to processing components identical to those shown in FIG. 1 that is referred to in relation to an embodiment to be described later. FIG. 6 is not intended to unnecessarily limit the present invention.
An example of actions to be performed in the power amplification system of the example will be described below.
The delay circuit 1 delays an inputted modulating signal by a predetermined time in the process of transferring the signal to the power amplifier 2. The power amplifier 2 amplifies the input signal, which has passed through the delay circuit 1, and outputs it.
The modulating signal is also inputted to the envelope detector 101. The envelope detector 101 detects an envelope of the modulating signal, and outputs it to the error amplifier 21.
The detected envelope is inputted to the error amplifier 21. The error amplifier 21 amplifies by a predetermined gain an error between the envelope and the supply voltage for the power amplifier 2, and outputs the resultant error.
An output terminal of the error amplifier 21 and a supply voltage terminal of the power amplifier 2 are linked by the resistor 22. The pulse width modulator 23 generates a pulse width modulated signal according to a potential difference across the resistor 22, and outputs the pulse width modulated signal to the pulse amplifier 24.
The pulse amplifier 24 amplifies the pulse width modulated signal inputted from the pulse width modulator 23, and outputs the resultant signal to the coil 25.
The coil 25 links an output terminal of the pulse amplifier 24 and the supply voltage terminal of the power amplifier 2.
In the circuitry of the example, a supply voltage to be applied to the power amplifier 2 is varied in line with the envelope of the inputted modulating signal. For a current of a low-frequency component as well as a direct current, the voltage is fed from the highly efficient pulse amplifier 24. For a current of a high-frequency component, the voltage is fed from the error amplifier 21. Thus, the efficiency of the power amplifier 2 is upgraded.
Referring to FIG. 7A and FIG. 7B, the principles of operation of the envelope tracking type power amplification system in accordance with a related art will be described below.
In FIG. 7A, the axis of ordinates indicates a current outputted from the error amplifier 21, and the axis of abscissas indicates elapse of time. In FIG. 7B, the axis of ordinates indicates a current outputted from the pulse amplifier 24, and the axis of abscissas indicates elapse of time.
To begin with, a description will be made of a case where a direct-current (DC) component is inputted from the envelope detector 101 to the error amplifier 21.
When a DC voltage is inputted from the envelope detector 101, the error amplifier 21 feeds a current to the power amplifier 2 so as to output a voltage obtained by amplifying the input by a predetermined gain (time point a in FIGS. 7A and 7B).
Accordingly, the potential difference across the resistor 22 increases, and the pulse width modulator 23 activates the pulse amplifier 24. When the pulse amplifier 24 is activated, a current is fed to the power amplifier 2 through the coil 25. The current fed from the error amplifier 21 diminishes (an interval b in FIGS. 7A and 7B).
If the current to be fed from the error amplifier 21 diminishes, the potential difference across the resistor 22 decreases. When the potential difference becomes as small as a predetermined potential difference, the pulse width modulator 23 inactivates the pulse amplifier 24 (an interval c in FIGS. 7A and 7B).
If the pulse amplifier 24 is inactivated, the current to be fed to the power amplifier 2 through the coil 25 diminishes. The current to be fed from the error amplifier 21 increases. When the predetermined potential difference is attained, the pulse width modulator 23 activates the pulse amplifier 24 again (an interval d in FIGS. 7A and 7B).
By repeating the foregoing actions, a necessary current is fed to the power amplifier 2.
If the DC voltage is inputted, a current is fed mainly from the pulse amplifier 24. Out of a signal outputted from the pulse amplifier 24, a high-frequency component that cannot be removed by the coil 25 is compensated by the error amplifier 21.
In the example shown in FIGS. 7A and 7B, a hysteresis is preserved between the potential difference across the resistor 22 attained when the pulse amplifier 24 is activated, and the potential difference across the resistor 22 attained when the pulse amplifier 24 is inactivated.
Next, a description will be made of a case where an alternating-current (AC) voltage is inputted from the envelope detector 101 to the error amplifier 21.
An oscillation frequency of the pulse width modulator 23 is determined with a width of hysteresis or an inductance of the coil. For a frequency which the pulse width modulator 23 can follow, the power amplification system operates under the same principles as the principles of operation for the DC voltage.
For all frequency components the pulse width modulator 23 cannot follow, a current is fed from the error amplifier 21.
Prior art reference JP-A-2003-092518.